Heretofore, known as quick drying printing ink have been ink compositions for actinic radiation curable ink-jet ink (hereinafter also referred to as ink compositions), which undergo polymerization via exposure to actinic radiation such as ultraviolet radiation, resulting in ink curing. The above radically polymerizable ink compositions have exhibited problems in which curing is retarded in the presence of oxygen.
In recent years, proposed have been ink compositions which undergo cationic polymerization via exposure to actinic radiation. Proposed as above cationically polymerizable ink compositions have been those which are composed of oxetane compounds, cationic photopolymerization initiators, and pigments, and if desired, to which epoxy compounds are added (refer, for example, to Patent Document 1). A cationic polymerization reaction is initiated in the presence of cations generated from cationic polymerization initiators via exposure to ultraviolet radiation. Consequently, polymerization is not hindered by oxygen, and specifically, no practical limit exists in which it is inevitable to perform the reaction in an inactive ambience. Advantages are exhibited in which it is possible to quickly perform the reaction in air and to achieve excellent polymerization.
However, when cationically polymerizable compounds such as oxetane compounds or epoxy compounds are stored for an extended period of time in a state in which cationic polymerization initiators are incorporated, cations may be formed from the above cationic polymerization initiators even though ultraviolet radiation is shielded. Thereby, problems have occurred in which cationically polymerizable compounds are subjected to polymerization initiation, resulting in an increase in viscosity and gelling.
Proposed as one method to overcome the drawbacks described above is a method (refer, for example, to Patent Document 2) in which one or at least two types of guanidine based compounds, thiazole based compounds, benzothiazole based compounds, thiazole carboxylic acid based compounds, sulfenamide based compounds, thiourea based compounds, imidazole based compounds, benzimidazole based compounds, and alkylphenyl sulfide based compounds are incorporated in a composition composed of cationic polymerization catalysts and cationically polymerizable organic materials, both being essential components, whereby it is possible to improve stability at room temperature (being pot life) of the above compositions.
Further, methods are proposed (refer, for example, to Patent Documents 3-5) in which basic compounds such as hydroxides or carbonates of alkali metals or alkali earth metals, basic inorganic compounds such as alcolates, basic organic compounds such as amine or other nitrogen containing basic compounds, or water are incorporated in oxetane compounds, whereby storage stability is improved without degrading ring opening polymerization properties.
However, when cationic polymerization initiators are incorporated in cationically polymerizable compounds, the addition of the basic compounds, described above, does not sufficiently improve storage stability. Specifically, in the case of a mixed system in which cationically polymerizable compounds are blended with oxetane compounds and alicyclic epoxy compounds, gel is formed due to the fact that the reactivity of the oxetane compound is markedly enhanced in the presence of cationic polymerization initiators, compared to the case in which oxetane compounds are individually present. As a result, at present, it is not possible to obtain sufficient improved effects for storage stability.
Further, in order to overcome the drawbacks as described above, an increase in the added amount of the above guanidine based compounds and basic compounds overcomes the drawback of gelling, while a further drawback in which curability is degraded occurs, whereby it has been difficult to simultaneously achieve both storage stability and curability. Further, some of guanidine based compounds and basic compounds, described above, are harmful. In view of adaptability to environment, it is not preferable to incorporate such harmful substances in a large amount in composition.                (Patent Document 1) JP A No. 8-143806 (claims and examples)        (Patent Document 2) JP-A No. 8-283320 (claims and examples)        (Patent Document 3) JP-A No. 2000-186079 (claims and examples)        (Patent Document 4) JP-A No. 2000-327672 (claims and examples)        (Patent Document 5) JP-A No. 2003-252979 (claims and examples)        